Numerous methods exist in semiconductor manufacture for chemical etching and for cleaning of semiconductor wafers. One category of such methods is chemical solvent cleaning. However, it is recognized in the art that the use of environmentally hazardous chemicals, such as solvents used in chemical solvent cleaning, is disfavored. Thus, methods other than chemical solvent cleaning have been developed.
Key among non-solvent methods is the use of plasmas that form reactive ionized molecules that chemically etch a wafer. Plasma-based methods are used, for example, for etching and for post-process cleaning. One example of post-process cleaning is the stripping of photoresist following etching processes. Among these cleaning methods are those that utilize a reactive ion etching (“RIE”) process at low temperature, either by itself or in combination with a microwave source. An RIE process combines plasma and ion beam to achieve removal of a surface layer. For example, U.S. Pat. No. 5,908,319 discloses methods in which a microwave source associated with a gas inlet forms a reactive gas comprising free radicals. A radio frequency (“RF”) source for the RIE, positioned closer to a wafer being treated, ionizes the free radicals of the microwave plasma. The resulting RIE ions contact the wafer to achieve a desired reaction. This reference and all patents, patent applications, patent publications, and all other publications cited herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually set forth in its entirety.
Despite common use of RIE processes for photoresist stripping, it is known that the use of an RF plasma source for the RIE can lead to plasma damage. Such damage is known in the art, and is discussed in “Microchip Fabrication,” 3rd Edition, Peter Van Zant, pp. 269, 277; 1997 (which is incorporated by reference for this and other aspects of wafer fabrication). Depending on the concentrations and energy levels of energetic atoms, radicals, ions, electrons and photons in a plasma, plasma-caused damage may include surface leakage, damage to silicon, degradation of films (especially oxides), and changes in electrical parameters.
For example, a region of high voltage plasma bombards excess ions to the wafer surface, which can result in charging of exposed metal conductors. If the total surface area of conductors connecting to a gate is too large, the RIE plasma-induced ions striking this surface area then lead to a current discharge through the relatively thin gate oxide, which can lead to permanent damage.
While plasma damage generally is recognized to be of a greater concern during etching, where metal conductors (“antennas”) are exposed for relatively long periods compared to, for instance, plasma enhanced deposition, it has been learned that RIE plasma also is problematic in photoresist removal and via clean steps.
Regarding the latter, there is a need for a non-chemical solvent, plasma cleaning process that is characterized by less plasma damage to the wafer than is associated with RIE plasma. The methods of the present invention meet this need, and are particularly suited for use following formation of vias.